The overall objective of this investigation is to define the regulatory mechanisms that determine bone cell proliferation, specialization and function. We will pursue this objective by studying isolated bone cells dispersed from well differentiated bones. Cells so isolated have already been shown to perform specialized functions and to respond to low concentrations of hormones which modify skeletal structure. We will extend ongoing studies in order to: 1) characterize the hormonal and non-hormonal control of bone cell multiplication and specialization using a serum-free culture system that appears to promote cell division and selective growth of osteoblast-like cells, 2) characterize and purify a putative bone cell multiplication stimulating factor obtained from incubation medium conditioned by cultured bone cells, 3) refine and generate methods for obtaining individual bone cell populations (e.g.: obsteoblasts from mixed populations) and 4) explore the process whereby bone cells become relatively refractory to some agents that enhance adenylate cyclase activity. The availability of the serum-free culture system should allow the establishment of criteria for bone cell proliferation and maturation, and experimental assessment of the thesis that parathyroid hormone, vitamin D metabolites, calcitonin and other hormones modify these processes at both physiological and pharmacological concentrations. Use of conditioned media may well facilitate the development of a clone culture system for bone cells that can be used to study the specialization of specific cell types. These studies, taken together, could form a basis for understanding the cellular derangements that underlie skeletal disease.